The serialized lower is the part that you list on your ATF form 1.
However, you do list the caliber, barrel length, and overall length, so you may need to contact ATF before replacing the upper with an upper that is a different size or caliber.

BATFE recognizes the AR15 platform is modular so they allow temporary use of caliber/length other than that listed on the Form 4. However, if you are making a permanent change, you need to amend the Form 4 with the updated information. This does not need a separate application or tax stamp, just a letter to the NFA Branch.

Cool, thats what i was thinkin. I dont much deal with class 3 NFA, so i figured it best to ask.

The upper is my BIL's. His dad gave him an early pre M4 AR15 that his grandad bought back in the 60s. It is fit with a 5.5" flash hider, unpined to the barrel. The upper is clean and hardly used. He brought it over to me so I could check it over and clean it for him. That was when I noticed it was an SBR. Its an A1 upper with a 12" barrel, threaded 1/2X28, and im pretty sure its a colt.

He is wanting to swap it for a 16" carbine flat top upper.

Here I took a couple pics of it. In case anyone here would be interested in a swap.